I used a script to install ImageMagick http://github.com/masterkain/ImageMagick-sl
After a while, I got ImageMagick installed.
Then I ran sudo gem install rm
On Mac Os X, check if /opt/local/bin is in your PATH. If not:
/opt/local/bin
PATH
export PATH=/opt/local/bin:/opt/local/sbin:$PATH sudo gem install rmagick
This worked for me!